Controllable current source

ABSTRACT

A circuit for providing a current the magnitude of which is determined by an externally generated control signal and which is substantially independent of the impedance of the load to which the current is applied. A current generating network generates an output current in accordance with the algebraic sum of the control signals applied to the inputs thereof. One of these inputs is energized by an externally generated control signal which sets the desired value of output current. A second of these inputs is controlled in accordance with a feedback signal indicative of the output current to assure that attempted deviations of the output current from the desired value are counteracted before they substantially affect the level of current through the load.

United States Patent [191 Kiko Mar. 11, 1975 CONTROLLABLE CURRENT SOURCE[75] Inventor: Frederick Kiko, Sheffield Village, PrimaryExamlfler-Roberi Schaefer Ohio Assistant Examiner-Mr Ginsburg Attorney,Agent, or Firm-Edward C. Jason [73] Assignee: Lorain ProductsCorporation,

Ohlo' 57 ABSTRACT [22] Flled: Sept- 1131973 A circuit for providing acurrent the magnitude of 211 App] 39 225 which is-determined by anexternally generated control signal and which is substantiallyindependent of Related Apphcatlon Data the impedance of the load towhich the current is up- [62] Division of Ser. No. 301,968, Oct. 30,1972, plied. A current generating network generates an outabandoned putcurrent in accordance with the algebraic sum of the control signalsapplied to the inputs thereof. One U-S. Cl. of these inputs is energizedan externally generated III. C. onni ignal ets the desired value ofutput Field M Search 323/1, 16, 19; current. A second of these inputs iscontrolled in ac- 307/31, 33, 35 cordance with a feedback signalindicative of the output current to assure that attempted deviations ofthe References Cited output current from the desired value are counter-UNITED STATES PATENTS acted before they substantially affect the levelof cur- 3,508.08l 4/1970 Matsuda 323 1 UX fem through the load-3.564.444 2/l97l Walsh 323/4 UX 3.566.246 2/1971 Seer 323/4 8 Clams 3D'awmg figures .J CONTROL. sourzcE I PATEHTEDHARI 1 5 2.870.896

sumlrfz CONTROL. SOURCE CONT IZOL SOURCE CONTROLLABLE CURRENT SOURCEThis is a division of application Ser. No. 301,968, filed Oct. 30, 1972,now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to currentsources and is directed more particularly to current sources having anelectronically controllable output current.

Current sources, that is, sources which produce currents the magnitudesof which are substantially independent of the input impedances of thecircuits connected thereto, are utilized in a wide variety of electricand electronic control circuits. Current sources are, for example, usedto supply bias currents which are substantially independent oftemperature drift or induced voltages in. the circuitry being biased.Current sources are also utilized to charge capacitors which must have alinearly increasing terminal voltage. Current sources have also beenused to limit the amount of power which can be drawn from a source whichdrives a load that is subject to being short-circuited. Numerous otherusages are familiar to those skilled in the art.

Presently available current sources commonly present one of twoproblems. On the one hand, current sources which are simple andinexpensive present the problem of providing an output current which isnot sufficiently independent of changes in the impedance of the loadconnected thereto to be suitable for use in precise control systemapplications. On the other hand, current sources, heretofore known,which do produce a sufficiently constant output current are complex andexpensive. In addition, current sources commonly present the furtherproblem that the level of current provided thereby cannot be varied inany convenient manner with an externally generated control signal.

In accordance with'the present invention, there is provided a currentsource which is simple and economical, which maintains the desired levelof output current with a high degree of accuracy and which can becontrolled in accordance with an externally generated control signal, asrequired, to change the magnitude of the output current.

SUMMARY OF THE INVENTION It is an object of the invention to provide animproved current source.

Another object of the invention is to provide an improved current sourcewhich accurately maintains the desired level of output current as theimpedance of a load supplied therefrom varies over a wide range.

It is another object of the invention to provide a current source havinga novel feedback network whereby the output current is maintained at thedesired value in spite of changes in the impedance of a load connectedthereto.

Still another object of the invention is to provide an improved currentsource of the above character wherein the output current can becontrolled in accordance with an externally generated control signal.This allows the circuit of the invention to provide a timevarying outputcurrent having an instantaneous magnitude which is substantiallyindependent of the impedance of the load connected thereto.

Yet another object of the invention is to provide an improved currentsource of the above character lending itself to either direct or inversevariation with changes in the magnitude of an externally generatedcontrol signal.

It is another object of the invention to provide an improved currentsource which provides a first output current that varies directly with acontrol signal and a second output current that varies inversely withthe same control signal.

It is still another object of the invention to provide an improvedcurrent source of the above character which may be energized by twocontrol sources simultaneously to provide two equal and opposite outputcurrents which vary in accordance with the difference between thecontrol signals provided by the two different controls sources.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of oneexemplary circuit embodying the invention,

FIG. 2 is a schematic diagram of another exemplary circuit embodying theinvention, and

FIG. 3 is a schematic diagram of yet another circuit embodying theinvention.

DESCRIPTION OF THE INVENTION Referring to FIG. 1 there is shown acurrent source of the invention for energizing a load 10 with a currentwhich varies substantially only in accordance with the magnitude of acontrol voltage or current provided by a control source 12. In thepresent illustrative embodiment, the circuit of the invention includesload current generating means which here takes the form of anoperational amplifier 14 having an inverting input 14a, a non-invertinginput 14b and an output 140. This operational amplifier serves togenerate and apply to load 10 a current having a magnitude which variesinversely in accordance with the algebraic sum of the signals that areapplied to the input 14a thereof through resistors 15a and 15b. Thecircuit of the invention also includes current sampling means, whichhere takes the form of a resistor 16, for providing a voltage thatvaries directly in accordance with the current which amplifier l4establishes through load 10. The circuit of the invention also includesa feedback network 18 for sensing the voltage established by currentsampling resistor 16 and for controlling the feedback signal applied toamplifier input 14a, as required, to cause the current through load 10to be substantially independent of the impedance thereof.

It will be understood that the terms directly and non-inverting are usedherein to describe conditions in which a positive change in one voltageor current produces a positive change in another voltage or current.These terms are to be distinguished from the terms inversely andinverting which are used herein to describe conditions. in which apositive change in one voltage or current produces a negative change inanother voltage or'current. It will be further understood that neitherterm requires that the magnitude of the change in one voltage or currentis linearly related to the magnitude of the change in the other voltageor-current.

To the end that feedback network 18 may generate a feedback signalsuitable for application to amplifier 14, network 18 includes anoperational amplifier 20 having an inverting input 20a, a noninvertinginput 20b and an output 200. Feedback network 18 also includes amplifierinput resistors 22a and 22b, a feedback resistor 24 and an amplifierbalancing resistor 26. This resis-. tor-amplifier configurationproduces, between network output 18a and ground G, a voltage whichvaries directly in accordance with the magnitude of the current flowingin the positive or rightward directionthrough current sampling resistor16.

The operation of the circuit of the invention will now be described.Assuming that the voltage which control source 12 applies to circuitinput terminal 12a and the current which flows out of circuit outputterminal a are at the desired values, feedback networkl8 applies toamplifier input 14a a feedback signal which is just sufficient to causeamplifier 14 to maintain the level of current flow through load 10 atthe desired value. Under these conditions, the circuit of FIG. 1 is in astable or quiescent state since no voltage or current exists which tendsto change the existing conditions of the circuit. 1

If, under the above conditions, the resistance of load 10 shoulddecrease, the current through sampling resistor 16 will increase. Thisincrease in current, in turn, increases the positive voltage betweenfeedback network output 18a and ground G and thereby increases themagnitude of the positive voltage at amplifier input 14a. Since apositive change in the voltage at amplifier input 140 causes a negativechange in the voltage at amplifier output 14c, the above increase in thevoltage at input 14a causes the output voltage of amplifier 14 todecrease and thereby restore the current through sampling resistor 16and load 10 to its quiescent value.

If, on the other hand, the resistance of load 10 should increase, thecurrent through resistor 16 will decrease. This decreases the positivefeedback signal which network 18 applies to amplifier input 14a andthereby increases the output voltage of amplifier 14. The latterincrease, in turn, restores the current through current samplingresistor 16 and load 10 to its quiescent value. Thus, departures in thecurrent through resistor 16 from its quiescent value give rise tofeedback signals which change the output voltage of amplifier 14, asrequired, to return the current through resistor 16 and load 10 to itsquiescent value.

Because amplifier 14 is responsive to the algebraic sum of the signalsapplied to the input thereof, both the feedback signal provided bynetwork 18 and the control voltages or currents applied through inputresistors such as 15a determine the magnitude of the current throughsampling resistor 16 and load l0.-Since, as previously described, thefeedback signal of network 18 serves to maintain the output current atits quiescent value, control of the quiescent value itself must bedetermined by the applied control voltages or currents. Accordingly, ifthe voltage or current of source 12 is constant, the magnitude of thecurrent flow through sampling resistor 16 and load 10 will be constant,and, if the voltage or current of source 12 varies with time, thequiescent current through sampling resistor 16 and load 10 will vary ina similar fashion with time. Under the latter conditions, network 18will vary the feedback signals, as required, to maintain the outputcurrent at the value determined by source 12, in spite of variations inthe impedance of load 10. Thus, whether the current through samplingresistor 16 and load 10 is constant or time-varying, the circuit of theinvention acts as a current source, that is, provides an output currentwhich is substantially independent of the imped ance of the load appliedthereto.

Because the control signal established by control source 12 isapplied-t0 the inverting input of amplifier 14, a positive change in thevoltage between circuit input terminal 12a and ground G causes anegative change in the magnitude of the current flowing to the right outof circuit output terminal 10a. Similarly, a negative change in thevoltage between circuit input 12a and ground G causes a positive changein the current flowing to the right out of circuit output 10a. Thus, thecircuit of-FIG. 1 comprises an inverting type current source.

If it is desirable to produce a current source wherein the current atcircuit output 10a varies directly rather than inversely with thecontrol signal at circuit input 12a, this may be'accomplished byutilizing the circuit of FIG. 2 which is-similar to the circuit of FIG.I, like parts being similarly numbered.

The circuit of FIG. 2 differs from the circuit of FIG. 1 in thattheinverting and non-inverting inputs of operat io nal amplifier 14 andoperational amplifier 20 are interchanged. The interchange of theseamplifier inputs does not alter the previously described currentregulating characteristic since the effect of a feedback signal whichvaries inversely with output current has the same effect on thenon-inverting input of an operational amplifier as has a feedback signalwhich varies directly with output current on the inverting input of anoperational amplifier. Thus, the operation of the circuit of FIG. 2 isidentical to the operation of the circuit of FIG. 1, except that theoutput current of the circuit of FIG. 2 varies directly with the voltageof source 12 while the output current of the circuit of FIG. 1 variesinversely with that voltage.

Under circumstanceswhere it is neccessary for a current source to supplycurrent to a balanced load as, for example, a two-wire transmissionline, it is desirable to energize the latter with a current sourcehaving two balanced current outputs, that is, a current source whichprovides two equal but opposite output currents. One form of theinvention which is suitable for providing such balanced output currentsis shown in the circuit of FIG. 3.

Referring to FIG. .3, there is shown first or negative load currentgenerating means which here takes the form of an operational amplifier28 and second or positive load current generating means which here takes'the form of an operational amplifier 30. Amplifier 28 energizes a firstload Ll with a current which varies inversely with the voltage ofcontrol source 12. Similarly, amplifier 30 energizes a second load L2with a current which varies directly with the voltage of control source12. These currents are supplied to loads L1 and L2 through respectivecurrent sampling resistors 32 and To the end that the negative andpositive load current supplied by amplifiers 28 and 30, respectively,may be of equal amplitude, amplifier 30 is provided with input andfeedback resistors 36 and 38, respectively, these resistors being soproportioned that the output voltage of amplifier 30 is equal inamplitude to the output volt age of amplifier 28. In addition,inputresistor 36 is connected to the output of amplifier 28 and to theinverting input of amplifier 30 to assure that the output voltages ofamplifiers 28 and 30 have opposite signs. Ac-

cordingly, it will be seen that the output voltages of amplifiers 28 and30 are equal in amplitude and opposite in'sign. It will be understoodthat the equal and opposite voltages generated by separate amplifiers 28and 30 may also be generated by'a single operational amplifier of thetype which generates two complementary output voltages.

In order that the currents in balanced loads L1 and L2 may be determinedby control source 12 and not by the impedances of loads L1 and L2, thereis provided a feedback network including a first component feedbacknetwork 40a and a second component feedback network 40b. These networksenergize the inverting input of amplifier 28 with a plurality offeedback currents which add togetherto provide a net feedback signalthat is proportional to the sum of the current through current samplingresistors 32 and 34. Resistors 42 and 44 of feedback network 40b, forexample, energize the input of amplifier 28 with feedback currents thatvary with the potentials with respect to ground of sampling resistorterminals 32a and 3412, respectively. Similarly, resistors 46 and 48 offeedback network 40a serve to couple to the input of amplifier 28feedback currents that vary with the potentials with respect to groundof sampling resistor terminals 32b and 34a.

Tothe end that the net feedback current applied to the input ofamplifier 28 may vary with the sum of the load currents through samplingresistors 32 and 34, resistors45 and 48 are connected to amplifier 28through an amplifier 50 which inverts the phase of the feedback currentsthrough fesistors 46 and 48. This inversion assures that only thedifference between the sum of the currents in feedback resistor 4244 andthe sum of the currents in feedback resistors 46-48 is effective incontrolling amplifier 28. It will be understood that for the differencebetween the above feedback currents to accurately reflect the sum of thecurrents through resistors 32 and 34, resistors 52 and 54 must havevalues such that the common components of the voltages at terminal pairs32a-32b and 34a-34b substantially cancel one another under no loadconditions.

Assuming that the above described resistance values are substantially asdescribed, feedback networks 40a and 40b will operate in the mannerdescribed in connection with the circuits of FIGS. 1 and 2 to render thecurrents through loads Ll and L2 substantially independent of theimpedance thereof. That is, the feedback network comprising componentfeedback networks 40a and 40b provides amplifiers 28 and 30 with afeedback signal which opposes any attempt of the current through loadsL1 and L2 to deviate from the value set by control source 12.

As previously described, the circuit of FIG. 3 generates in load Ll acurrent which varies inversely with the voltage of source 12 and in loadL2 a current which varies directly with the voltage of source 12. If itis desirable to on occasion reverse the phase relationship between thecurrents in loads L1 and L2 and the voltage of source 12, this may beaccomplished by switching control source 12 from contact with inputterminal 12a and into contact with an alternative or phase-reversinginput terminal 12b. This is because the latter terminal allows controlsource 12 to energize the input of amplifier 28 through a conductor 56and inverting amplifier 50 and thus effectively reverse the phase of thecontrol signal before it is introduced into the current controlcircuitry of FIG. 3. Thus, the application of a control signal to input12b causes the current in load L1 to vary directly with the controlsignal and causes the current in load L2 to vary inversely with thecontrol signal.

In addition to serving as alternative inputs to the current source ofFIG. 3, inputs 12a and 12b may be energized simultaneously from separatecontrol sources. In the latter event, it will be understood that thecurrents in load L1 and L2 will vary with the difference between thecontrol signals applied to inputs 12a and 12b. Thus, the circuit of FIG.3 is suitable for use as a current source having equal and oppositeoutputs and having a differential input.

In view of the foregoing it will be seen that a current sourceconstructed in accordance with the prescnt invention is adapted toprovide an output currcnt which varies in accordance with an externallygenerated control signal and is adapted to maintain that current at thedesired value substantially without regard to the impedance of the loadto which the current is applied. Furthermore, the circuit of theinvention is readily adaptable to forms in which this variation iseither direct or inverse or both direct and inverse.

It will be understood that the above described embodiments are fordescriptive purposes only and may be changed or modified withoutdeparting from the spirit and scope of the appended claims.

What is claimed is:

1. In a circuit for generating two currents which are substantiallyequal and opposite and which have magnitudes that are substantiallyindependent of the impedance of the circuitry to which the currents areapplied, the combination of, a circuit input, first and second circuitoutputs, first and second current generating means each having invertinginput means and output means, first and second current sampling meansfor generating respective signals which vary in accordance with thecurrents at respective circuit outputs, means for connecting the inputmeans of said first generating means to said circuit input, means forconnecting said first sampling means between the output means of saidfirst generating means and said first circuit output, means forconnecting the input means of said second generating means to the outputmeans of said first generating means, means for connecting said secondsampling means between the output means of said second generating meansand said second circuit output, feedback means for energizing the inputmeans of said first generating means with a feedback signal indicativeof the current through both said first and said second sampling meansand means for connecting said feedback means to said first and secondsampling means and to the input means of said first generating means.

2. A circuit as set forth in claim 1 including a phasereversing circuitinput, inverting means having input means and output means, saidinverting means serving as means for generating a signal at the outputmeans thereof which is substantially equal and opposite to the signal atthe input means thereof, and means for connecting said inverting meansbetween said phasereversing circuit input and the input means of saidfirst current generating means.

3. In a circuit for generating two currents which are substantiallyequal and opposite and which have magnitudes that are substantiallyindependent of the impedance of the circuitry to which the currents areapplied, the combination of, first and second circuit inputs, first andsecond circuit outputs, current generating means having input means andfirst and second output means, said current generating means serving asmeans for establishing substantially equal and opposite voltages at theoutput means thereof in accordance with the voltage at the input meansthereof, first and second current sampling means, means for connectingsaid current sampling means betweenrespective output means of saidcurrent generating means and respective circuit outputs, means forconnecting the input means of said current generating means to saidfirst and second circuit inputs, feedback means for energizing the inputmeans of said current generating means in accordance with the currentthrough both said first and said second current sampling means and meansfor connecting said feedback means to said sampling means and to theinput means of said generating means.

4. A circuit as set forth in claim 3 in which the means for connectingthe input means of said generating means to said first and secondcircuit inputs includes a connector for connecting the input means ofsaid generating means to said first circuit input and inverting meansfor connecting the input means of said generating means to said secondcircuit input.

5. A circuit as set forth in claim 3 in which said feedback meansincludes first, component feedback means for providing an invertedfeedback signal and second, component feedback means for providing anoninverted feedback signal.

6. In a circuit for generating two currents which are substantiallyequal and opposite and which have magnitudes that are substantiallyindependent of the impedance of the circuitry to which the currents areapplied, the combination of, a circuit input, first and second circuitoutputs, current generating means having input means and first andsecond output means, said current generating means serving as means forestablishing substantially equal and opposite voltages at said first andsecond output means in accordance with the voltage at the input meansthereof, first and second current sampling means,.means for connectingsaid first and second current sampling means in current sensingrelationship between respective output means of said current generatingmeans and respective circuit outputs, means for connecting the inputmeans of said current generating means to said circuit input, feedbackmeans for energizing the'inputmeans of said current generating means inaccordance with the currents sensed by both of said current samplingmeans and means for connecting said feedback means to said samplingmeans and to the input means of said current generating means.

7. A circuit as set forth in claim 6 in which said feedback meansincludes first, second, third and fourth resistance means, and in whichsaid means for connecting said feedback means to said current samplingmeans and to the input means of said current generating means includesmeans for connecting said first resistance means to a first end of saidfirst current sampling means and to the input means of said generatingmeans, means for connecting said second resistance means to a second endof said second current sampling means and to the input means of saidgenerating means, means for connecting said third resistance means to asecond end of said first current sampling means and to the input meansof said generating means and means for connecting said fourth resistancemeans to a first end of said second current sampling means and to theinput means of said generating means.

8. A circuit as set forth in claim 7 in which said connecting means forsaid third and fourth resistance means includes inverting means forinverting the phase of the feedback signals applied to said generatingmeans through said third and fourth resistance means.

1. In a circuit for generating two currents which are substantiallyeQual and opposite and which have magnitudes that are substantiallyindependent of the impedance of the circuitry to which the currents areapplied, the combination of, a circuit input, first and second circuitoutputs, first and second current generating means each having invertinginput means and output means, first and second current sampling meansfor generating respective signals which vary in accordance with thecurrents at respective circuit outputs, means for connecting the inputmeans of said first generating means to said circuit input, means forconnecting said first sampling means between the output means of saidfirst generating means and said first circuit output, means forconnecting the input means of said second generating means to the outputmeans of said first generating means, means for connecting said secondsampling means between the output means of said second generating meansand said second circuit output, feedback means for energizing the inputmeans of said first generating means with a feedback signal indicativeof the current through both said first and said second sampling meansand means for connecting said feedback means to said first and secondsampling means and to the input means of said first generating means. 1.In a circuit for generating two currents which are substantially eQualand opposite and which have magnitudes that are substantiallyindependent of the impedance of the circuitry to which the currents areapplied, the combination of, a circuit input, first and second circuitoutputs, first and second current generating means each having invertinginput means and output means, first and second current sampling meansfor generating respective signals which vary in accordance with thecurrents at respective circuit outputs, means for connecting the inputmeans of said first generating means to said circuit input, means forconnecting said first sampling means between the output means of saidfirst generating means and said first circuit output, means forconnecting the input means of said second generating means to the outputmeans of said first generating means, means for connecting said secondsampling means between the output means of said second generating meansand said second circuit output, feedback means for energizing the inputmeans of said first generating means with a feedback signal indicativeof the current through both said first and said second sampling meansand means for connecting said feedback means to said first and secondsampling means and to the input means of said first generating means. 2.A circuit as set forth in claim 1 including a phase-reversing circuitinput, inverting means having input means and output means, saidinverting means serving as means for generating a signal at the outputmeans thereof which is substantially equal and opposite to the signal atthe input means thereof, and means for connecting said inverting meansbetween said phase-reversing circuit input and the input means of saidfirst current generating means.
 3. In a circuit for generating twocurrents which are substantially equal and opposite and which havemagnitudes that are substantially independent of the impedance of thecircuitry to which the currents are applied, the combination of, firstand second circuit inputs, first and second circuit outputs, currentgenerating means having input means and first and second output means,said current generating means serving as means for establishingsubstantially equal and opposite voltages at the output means thereof inaccordance with the voltage at the input means thereof, first and secondcurrent sampling means, means for connecting said current sampling meansbetween respective output means of said current generating means andrespective circuit outputs, means for connecting the input means of saidcurrent generating means to said first and second circuit inputs,feedback means for energizing the input means of said current generatingmeans in accordance with the current through both said first and saidsecond current sampling means and means for connecting said feedbackmeans to said sampling means and to the input means of said generatingmeans.
 4. A circuit as set forth in claim 3 in which the means forconnecting the input means of said generating means to said first andsecond circuit inputs includes a connector for connecting the inputmeans of said generating means to said first circuit input and invertingmeans for connecting the input means of said generating means to saidsecond circuit input.
 5. A circuit as set forth in claim 3 in which saidfeedback means includes first, component feedback means for providing aninverted feedback signal and second, component feedback means forproviding a non-inverted feedback signal.
 6. In a circuit for generatingtwo currents which are substantially equal and opposite and which havemagnitudes that are substantially independent of the impedance of thecircuitry to which the currents are applied, the combination of, acircuit input, first and second circuit outputs, current generatingmeans having input means and first and second output means, said currentgenerating means serving as means for establishing substantially equaland opposite voltages at said first and second output means inaccordance with the voltage at thE input means thereof, first and secondcurrent sampling means, means for connecting said first and secondcurrent sampling means in current sensing relationship betweenrespective output means of said current generating means and respectivecircuit outputs, means for connecting the input means of said currentgenerating means to said circuit input, feedback means for energizingthe input means of said current generating means in accordance with thecurrents sensed by both of said current sampling means and means forconnecting said feedback means to said sampling means and to the inputmeans of said current generating means.
 7. A circuit as set forth inclaim 6 in which said feedback means includes first, second, third andfourth resistance means, and in which said means for connecting saidfeedback means to said current sampling means and to the input means ofsaid current generating means includes means for connecting said firstresistance means to a first end of said first current sampling means andto the input means of said generating means, means for connecting saidsecond resistance means to a second end of said second current samplingmeans and to the input means of said generating means, means forconnecting said third resistance means to a second end of said firstcurrent sampling means and to the input means of said generating meansand means for connecting said fourth resistance means to a first end ofsaid second current sampling means and to the input means of saidgenerating means.